Tricolored Blackbird Agelaius tricolor is a rapidly declining species largely endemic to California and forms larger breeding colonies than any other extant North American landbird following the extinction of Passenger Pigeon Ectopistes migratorius. We present information on its distribution, breeding habitat and changes in global population size using data collected since the 1930s. We also present data on reproductive success at 103 colonies between 1992 and 2003. While possibly once the most abundant bird throughout much of its range, it declined by over 50% between the 1930s and early 1990s, and by a further c. 56% between 1994 and 2000. The global population is now smaller than the historic size of some individual breeding colonies. Reproductive success was significantly higher in upland non-native vegetation (primarily Himalayan blackberry Rubus discolor) than in native emergent cattail Typha spp. and bulrush Scirpus spp. marshes, its likely predominant historic breeding habitat. Contemporary losses of important upland nesting substrate, combined with low reproductive success in native habitats and complete breeding failure in harvested agricultural fields, are the most likely causes of recent declines. Recovery of this species presents possible conflicts in conservation policy because successful reproduction now largely depends on invasive non-native plants and the willingness of farmers to delay harvest or to lose portions of their crops.
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